1. Field of the Invention
This invention relates generally to systems having an electric motor driving a mechanical apparatus and more particularly relates to the detection of mechanical loading transitions of the mechanical apparatus by monitoring motor current and is particularly useful as a backup system to conventional limit switches of other devices commonly used to detect the position of a component of the mechanical apparatus.
2. Description of the Related Art
There are many types of machines that transport people or move mechanical apparatus in the vicinity of people or otherwise require reliable control so they do not malfunction and cause personal injury or property damage. One of the most common electrical loads associated with such machines is an electric motor that is or drives a prime mover to move the mechanical apparatus. Such machines should not only operate when they are signaled or otherwise commanded to operate, but of more critical importance to safety is that they stop operating when they are signaled or otherwise commanded to stop. Although the invention is applicable to a broad variety of machines with electrical loads that have such control and safety requirements, it is illustrated in connection with one such machine, a wheelchair lift having an electric motor driven hydraulic pump as its prime mover.
Many buses and vans are equipped with hydraulic wheelchair lift systems. In wheelchair lift systems, safety is probably the single most important factor. These lifts transport people who have a physical disability and it is particularly desirable to avoid jeopardizing them with apparatus that has the possibility of failing and causing personal injury.
Typically, these lift systems consist of a platform that can be folded and unfolded between a vertically oriented, stowed position in the vehicle and an unstowed, transporting position horizontally extending from the vehicle floor. From its unfolded or unstowed position, the platform can be raised and lowered between the vehicle's floor level and the ground level like an elevator. The lift of FIG. 1 is a typical wheel chair lift system. Most such prior art lift systems use essentially the general principles that are illustrated. The lift allows a person in a wheelchair to roll along the ground and onto the lift platform to be raised into the vehicle. The platform is then raised from ground level up to the vehicle's floor level. After reaching the floor level, the person rolls from the platform into the vehicle. Then the person operates the mechanism to cause the platform to pivot into the vehicle and stow the lift in the vehicle.
To minimize the cost and complexity of a wheelchair lift system, it is advantageous to perform the platform lifting function and the stowing function utilizing a single hydraulic cylinder or two or more cylinders operated hydraulically in parallel, such as illustrated in FIG. 1. As known to those skilled in the art, the hydraulic cylinder can be located to either push or pull in order to raise the lift, depending upon which obliquely opposite pivots it is connected to in the parallelogram arrangement that supports the platform.
FIG. 2 shows the fundamental mechanical structures of a typical wheel chair lift system that incorporates a hydraulic cylinder 1 to perform both the wheelchair lowering and lift functions and the platform deployment and stow functions. The system includes a first fixed vertical pillar 2 that is securely attached to the vehicle. A lifting platform 3 is attached to a second, vertically movable, vertical pillar 4 at a hinging pivot 5. A brace 9 is attached between the vertical pillar 4 and the platform 3 in such a fashion as to limit the range of motion of platform 3 around hinging point 5 so that it can pivot to no more than a 90° angle to the vertical pillar 4. The vertical pillars 2 and 4 are mechanically coupled to each other with two parallel equal length arms 6 and 7 that are hinged at their attachment points to the vertical pillars 2 and 4. The hydraulic cylinder 1, when operated, raises the platform 3 from ground level up to vehicle floor level. Whenever the platform 3 is raised above floor level, a stop 8 engages a platform protrusion 3a which directs the motion of the platform 3 around its hinging point 5 causing the platform 3 to fold, that is to pivot upwardly about its pivot axis 5 near its innermost edge until it reaches a substantially vertical orientation.
This operation is illustrated in more detail in FIG. 3. A wheelchair lifting cycle begins, as illustrated in FIG. 3A, with the wheelchair lift system fully deployed so that the platform 3 is resting at ground level. In this position a wheelchair can easily be rolled on to or off of the platform. Pumping fluid into the hydraulic lifting cylinder 1 causes the second vertical pillar 4 and platform 3 to rise with respect to vertical pillar 2 from ground level towards the vehicle floor level as shown in FIG. 3B. The lifting cycle is completed when platform 3 reaches the vehicle's floor level as shown in FIG. 3C. In this position a wheelchair can easily be rolled between the pillars into or out of the vehicle.
Once the lift has served its purpose to raise the user to the vehicle floor level, the lift needs to be stowed. A stow cycle begins with platform 3 at vehicle floor level as illustrated in FIG. 3D. The mechanical structures are so arranged that after the platform reaches floor level, application of more force from the hydraulic cylinder causes the platform to pivot around its pivot point 5 because further vertical movement of the platform is limited by the floor level stop 8. Pumping fluid into the hydraulic cylinder causes the second vertical pillar 4 to rise with respect to vertical pillar 2 in turn forcing platform 3 to fold around pivot 5 as shown in FIG. 3E because the protruding part 3a of the platform 3 engages the stop 8, causing the platform to fold in against the pillars as the pillars 2 and 4 are driven together by the hydraulic cylinder, as shown in FIGS. 3D-3F. The stowing cycle is complete when platform 3 is fully recovered to its vertical stowed position as shown in FIG. 3F.
These operations are reversible. Releasing fluid from hydraulic cylinder 1 when platform 3 is in the fully stowed position, as shown in FIG. 3F, allows the force of gravity to first cause the second vertical pillar 4 to descend with respect to the first vertical pillar 2 allowing platform 3 to unfold around pivot 5. The unstow operation is complete when platform 3 is fully deployed and is parallel to and level with the vehicle's floor as shown in FIG. 3C. From this position a wheelchair can easily be moved from the vehicle onto the platform. Releasing additional fluid from the hydraulic cylinder 1 causes the second vertical pillar 4 and platform 3 to descend with respect to the first vertical pillar 2 from vehicle floor level to ground level. The platform lowering operation is complete when platform 3 reaches ground level as shown in FIG. 3A.
Turning now to the electrical and hydraulic circuitry, FIG. 4 illustrates a basic prior art hydraulic circuit and electrical controlling circuit for a wheelchair lift system described above although some conventional, prior art components and options are not included.
The hydraulic circuit includes a hydraulic lifting cylinder 11, an electric motor driven hydraulic pump 12, a normally closed, electrically energized, hydraulic fluid bypass valve 13 and a hydraulic fluid reservoir tank 14. A battery BAT is connected to a contactor 15 that operates as a power switch to control electrical current through the electric motor of the electric motor driven hydraulic pump 12. The electric motor is not directly switched on and off by a mechanical, hand-held switch because the motor currents are too large and would require an excessively large electrical cable in the user's hand to control the lift. So the separate contactor or power switch 15 is used. When electric power is applied to the hydraulic pump 12, fluid is pumped from the reservoir tank 14 to the lifting cylinder 11. Check valves internal to the hydraulic pump 12 prevent reverse hydraulic fluid flow through the pump. When power is applied to the bypass valve 13 and if the hydraulic lifting cylinder 11 is under pressure from a force applied to it, such as gravity, hydraulic fluid will return from the lifting cylinder 11 through the bypass valve 13 to the reservoir tank 14.
Low current switches 16, 17, 18, 19 and 20 control the power contactor 15. These include four separate hand control switches 17, 18, 19 and 20. Two of these switches, 17 and 18 can apply power to the contactor, closing its high current circuit and thereby applying current to the electrical motor to cause the motor to operate and develop hydraulic pressure for raising the lift. Two other switches 19 and 20 operate the bypass valve 13 causing fluid to drain from the hydraulic cylinder for its lowering movement. Each of the two sets of hand control switches is controlled by a fifth switch 16, and that fifth switch is mounted to the lift as a limit switch to be engaged and change state when the platform reaches the vehicle's floor level. Consequently, when the platform 3 is at ground level or at any intermediate position between the positions of FIG. 3A and 3C, switch 16 is in the state illustrated in FIG. 4. When the platform is rising and arrives at the position of FIG. 3C, the switch 16 switches to the opposite state and is in that state at every position above that.
There are four distinct functions performed by the wheelchair lift system described above which are:
1. Raising the platform
2. Stowing the platform
3. Deploying the platform
4. Lowering the platform
When the platform 3 is at ground level, switch 16 can supply power to switches 18 and 19. Switch 18 controls raising the platform. If platform 3 is below floor level, switch 16 connects the battery positive terminal to switch 18. Manually closing switch 18 connects the battery positive terminal to power contactor 15 in turn switching battery positive to apply battery voltage to the hydraulic pump 12. Unless switch 18 is opened, the hydraulic pump continues to operate until the platform reaches floor level at which time switch 16 changes state and removes battery power from switch 18 and the power contactor 15. When it does, the circuit to the contactor 15 through switch 18 is opened which interrupts the motor current and automatically stops the ramp at that level. At that point the user gets off the lift platform and then wants to stow the lift.
The user initiates stowing of the lift by pushing the stow button, to close switch 17 which controls stowing the platform. Manually closing switch 17 connects the battery positive terminal to power contactor 15 in turn switching battery positive to the electric motor of the hydraulic pump 12. The hydraulic pump operates raising the platform 3 from the vehicle floor level position to the fully stowed position at which time the switch 17 is manually released by the user. Of course a limit switch can be included to assure that the electric motor ceases operation.
Switch 20 controls deploying the platform. If platform 3 is above floor level, switch 16 connects the battery positive terminal to switch 20. Manually closing switch 20 connects battery positive to the hydraulic bypass valve 13 operating it to cause hydraulic fluid to drain from hydraulic cylinder 11 to reservoir tank 14. The hydraulic cylinder 11 retracts until the platform reaches floor level at which time switch 16 changes state and removes battery power from switch 20 and the hydraulic bypass valve 13.
Switch 19 controls lowering the platform from the vehicle floor level. Switch 16 connects the battery positive terminal to switch 19. Manually closing switch 19 connects battery positive to the hydraulic bypass valve 13 operating the valve 13 causing hydraulic fluid to drain from hydraulic cylinder 11 to the reservoir tank 14. The hydraulic cylinder 11 retracts until platform 3 reaches ground level or switch 19 is released.
Safety is the first consideration in the operation of any wheelchair lift system. Safe operation also depends on accurately sensing platform position in relation to vehicle floor level. The failure of any single component, switch, sensor or control should not affect safe operation. Examining the schematic in FIG. 4 reveals a potential safety problem. Switch 16 is the switch that changes from a first state to a second state when the ramp arrives at vehicle floor level. If position-sensing control switch 16 develops a mechanical failure of its mechanism that causes it to change states when it is not supposed to, or an electrical failure that its contacts would not change state, it is possible that the platform would not automatically stop at vehicle floor level during a lifting cycle but instead would immediately transition into a stowing cycle and rise right past floor level and toward a stowed position. Serious injuries could result to a person on the platform.
There are ways of dealing with the potential failure of switch 16. For example, two redundant switches can be used. Alternatively, there could be a light beam and light sensor to detect the presence of the platform at a location it should not be at particular places in the operating cycle. Redundant position-sensing control switches can increase reliability but they do so at the expense of increased cost and circuit complexity. Furthermore, what happens if the two redundant switches operate from the same cam and that cam fails? A light beam sensing system adds considerable expense and circuit complexity and provide additional structure that can be damaged during use and therefore disable the system and require repair.
It is therefore an object and feature of the invention to fill the need for an independent, low cost and reliable backup system to stop the lifting platform at floor level if the primary position-sensing switch or control circuit should fail.
A further object and feature of the invention is to provide a second system that monitors the same event but is not linked or interdependent in any way on the primary monitoring system so that a failure in one system could not possibly affect the second system.